1. Field of the Invention.
This invention is applicable to the treatment of municipal wastewater by the activated sludge process. It is particularly applicable to oxidation ditch activated sludge treatment systems which utilize a "race track" basin configuration. The invention can be incorporated into existing or new oxidation ditch systems to achieve improved settling of the activated sludge by providing selective pressure against the growth of filamentous bacteria, improved total nitrogen removal, energy savings, and recovery of alkalinity.
2. Description of Related Art.
a) Oxidation Ditch Activated Sludge Treatment Systems. The activated sludge process has been used for many years for the removal of biochemical oxygen demand (BOD) from municipal wastewaters. The process consists of an aeration basin containing a suspension of microorganisms referred to as mixed liquor. Wastewater is fed to the aeration basin and oxygen is utilized by the biomass to sorb, assimilate and metabolize the BOD available in the wastewater. From the aeration basin mixed liquor flows to a clarifier where the biomass settles and treated wastewater overflows. Most of the settled biomass is returned to the aeration basin. A smaller portion is wasted in order to maintain a relatively constant level of biomass in the system. The activated sludge process has been extensively described in the literature and in textbooks. Se Metcalf & Eddy.
Oxidation ditch activated sludge systems offer cost-effective wastewater treatment for small municipalities, and hundreds of such plants exist in the United States today. The process is characterized by an aeration basin that has an oval or racetrack configuration with unidirectional channel flow at sufficient velocity to maintain suspension and mixing of the biological solids or mixed liquor suspended solids (MLSS). Various types of mechanical equipment have been used in oxidation ditches to provide mixing and aeration. The most commonly used aeration systems are the brush rotor aerator which rotates partially submerged across the width of the channel, and the low speed surface aerator specifically located at the end of the dividing wall, as in the Carrousel system..
Other unique features of an oxidation ditch that distinguish it from conventional activated sludge treatment are:
1) a long solids retention time and hydraulic detention time of about 30 days and 24 hours, respectively;
2) a simplified plant flowsheet lacking primary clarification and anaerobic digestion; and
3) uses conservatively designed secondary clarifier loadings. These features result in a system that is easy to operate, provides a high quality effluent in terms of BOD and suspended solids, can handle variable loads well, and is economical for moderate and small size plants.
b. Nitrogen Removal in Oxidation Ditch Systems. Nitrogen removal in oxidation ditch systems is a function of the system design, plant loading, operational method, and aerator control method. Nitrogen removal efficiencies range from 60% to 95%, with aeration control and operational methods having a significant influence on performance (Randall et al., 1992). A description of oxidation ditch designs to accomplish nitrogen removal is provided in a recent Manual of Practice publication by the Water Environment federation (WEF, 1992). The manual points out that these channel flow systems may be operated to enhance nitrogen removal by carefully controlling dissolved oxygen (DO) levels in the basin. As the channel flow leaves the aeration zone in the oxidation ditch, the dissolved oxygen (DO) concentration decreases until it is depleted, and anoxic zones occur in the ditch where nitrate is used instead of DO to accomplish nitrogen removal. The location and size of these anoxic zones will vary with time due to diurnal loading changes, and will also depend on the average plant loading and design conditions. Consistent nitrogen removal by this method requires a comprehensive DO control system that includes channel DO measurements and a means to vary the aerator energy output. The latter is done by using two-speed motors, variable speed motors or variable level weirs. Under low loading conditions, it is very difficult, if not impossible, to accomplish nitrogen removal by this method since, for most designs, some aeration is needed to maintain channel flow. Variable nitrogen removal levels are obtained and vigilant control is needed to maximize performance. Nitrate consumption with this method is driven by the endogenous respiration rate of the channel mixed liquor, and not by the consumption of the influent BOD.
Another approach described requires the use of multiple oxidation ditches with a piping design that allows alternating the influent feed to the ditches, operation of the ditches in series, and alternating the effluent withdrawal from either of the ditches. This is referred to as the phased isolation ditch treatment or the Bio-denitro process. The ditches use internal mixers so that the aerators can be turned off, and the ditch can be operated as an anoxic tank during part of the operation. The following table summarizes a typical sequence. First the feed is directed to ditch 1 and the effluent leaves from ditch 2, and then these are switched. In the final two phases, the influent and effluent only travels through one of the two ditches under aerobic conditions to accumulate nitrate for use in the later anoxic operations.
TABLE 1 ______________________________________ EXAMPLE OF PHASED ISOLATION DITCH OPERATING SEQUENCE Feed Ditch 1 Ditch 2 Effluent ______________________________________ Ditch 1 Anoxic Aerobic Ditch 2 Ditch 2 Aerobic Anoxic Ditch 1 Ditch 2 Aerobic Aerobic Ditch 2 Ditch 1 Aerobic Aerobic Ditch 1 ______________________________________
Significant operator attention is required to determine when to alternate the feed point and ditch operating sequence and conditions. The operating sequence may not always be obvious to many plant operators. These systems have been used in Denmark, but have not been popular in the United States.
A third approach used to accomplish nitrogen removal with oxidation ditches is to build an external anoxic tank ahead of the oxidation ditch for contacting the influent wastewater with nitrate containing mixed liquor pumped forward from the oxidation ditch. This method provides consistent reliable nitrogen removal with its efficiency generally being a function of the mixed liquor pumping rate. However, the system requires a greater capital expenditure for the external anoxic tank and pumping system, as well as related energy costs for pumping and mixing in the anoxic tank. Such designs have been proposed in Florida to meet a 70% to 80% nitrogen removal level.
c) Bulking Sludge Control. Oxidation ditch systems generally develop poor settling sludge as indicated by high sludge volume index (SVI) values. The SVI is the volume in mL occupied by one gram of sludge after 30 minutes of settling of a 1.0 or 2.0 liter mixed liquor sample. The high SVI values are usually associated with significant levels of filamentous bacteria within the activated sludge floc. Oxidation ditch systems are operated with low organic loadings, and Jenkins et al. (984) have attributed such operating conditions to the development of a filamentous bacteria population. Filamentous bacteria have greater ability to scavenge organic substrates or oxygen, and thus they have an advantage in low loaded systems where substrate concentrations are minimal.
Albertson (1987) has reviewed developments that have led to design and operating strategies that can select against the development of filamentous bacteria. These conditions allow the non-filamentous bacteria to capture a large proportion of the incoming BOD, so that the non-filamentous bacteria grow instead of filamentous organisms. One such method is to establish anoxic conditions during the initial contact between the influent wastewater and the mixed liquor. Filamentous bacteria are ineffective in using nitrate for oxidation of the incoming BOD as compared to non-filamentous bacteria, and thus do not proliferate under such conditions. The commonly accepted method places an anoxic tank ahead of the aeration tank, with a high mixed liquor recycle rate from the aeration basin to the anoxic basin. This method was successfully demonstrated by Heide and Pasveer (1974) (See Robertson, p. 176) for control of SVI in an oxidation ditch. An SVI of 70 mL/g was achieved compared to an SVI of 500 mL/g when the influent wastewater was fed directly to the oxidation ditch. The disadvantages of this system were described above per its use for nitrogen removal.
d) Oxidation-Reduction Potential Control Methods. Oxygen-Reduction Potential (ORP) is a measurement of the ratio of oxidants to reductants in a system. For biological systems, ORP values are lower and become negative as oxygen is removed and nitrate is consumed. Peddie et al. (1990) showed that the change in ORP versus time could be used to determine when oxygen and nitrate were depleted after stopping aeration during operation of an aerobic sludge digester. During depletion of oxygen or nitrate, distinct changes in the slope of ORP versus time are observed. Wareham et al. (1993) showed that nitrogen removal could be improved during operation of a bench-scale aerobic digester with ORP control versus the use of a timer to turn the digester aeration on and off.
Charpentier et al. (1988) described the use of ORP control to regulate aeration of an oxidation ditch. They would decrease the aeration level if the ORP increased too much to indicate the formation of nitrate. In this way they could save energy by preventing the use of oxygen for the oxidation of ammonia. Thus, the purpose of employing ORP in this case was to prevent the formation of nitrate, and was not related to nitrogen removal.
e) Relevance of Prior Experience to the Process of This Invention. The only nitrogen removal scheme that also changes the ditch operation from aerobic to anoxic is the phased isolation ditch method. However, this method lacks a feedback control method as we disclose herein to determine the length of the anoxic period under varying conditions. The anoxic time could exceed the time needed for full nitrate depletion, for example. This method also requires multiple tanks and more complex piping and valves, whereas the method disclosed herein can be applied to a single tank oxidation ditch.
The related art that teaches the use of anoxic conditions for SVI control requires an external anoxic tank in contrast to our anoxic/aerobic operating cycles within the single ditch.
The related art on ORP control does illustrate the ability to interpret the ORP data to determine when nitrate is depleted in an aerobic digester. However, there is no relationship between this and our system disclosed herein to use the ORP probe with DO control for nitrogen removal and SVI control in treating wastewater in an oxidation ditch.